In multipoint conferencing, three or more endpoint terminals communicate with each other across a network. In the current telecommunications environment, multipoint conferencing is usually implemented over circuit-switched communication networks. Since connections are point-to-point in a circuit-switched network, a centralized resource, known as a multipoint control unit (MCU), is needed to link the multiple endpoints together. The MCU performs this linking by receiving multimedia (audio, video and/or data) information signals from endpoint terminals over point-to-point connections, processing the received information signals, and retransmitting the processed signals to selected endpoint terminals in the conference.
With the emergence of packet-based networks including local area networks, wide area networks and the Internet, multipoint conferencing is currently being defined for use over these packet-based networks. In packet-based networks, information is divided into blocks of data known as packets. Packets can be transmitted among endpoints using either unicast (i.e., one source to one destination) or multicast (one source to many destinations) transmission based on header information which contains appropriate unicast or multicast addressing information. Multicast is a technique that allows copies of a single packet to be transmitted to a selected subset of all possible destinations.
There are two methods known for multipoint conferencing using packet-based networks. These methods are referred to as "centralized" and "de-centralized" modes, respectively, in the draft ITU standard H.323 (ITU-T Draft Recommendation H.323: "Visual Telephone Systems and Equipment for Local Area Networks Which Provide a Non-Guaranteed Quality of Service", Jan. 30, 1996 incorporated herein by reference). In the centralized mode, endpoint terminals in a conference communicate with a centralized resource using unicast transmission. The centralized resource can be an MCU that provides control and processing functions to endpoint terminals in the conference. The MCU processing functions include mixing and switching of multimedia (audio, video and/or data) information streams received from the endpoints. In the centralized mode, the MCU unicasts these processed information streams to each endpoint terminal.
In the de-centralized mode, endpoint terminals multicast their multimedia information streams to all other endpoint terminals, rather than through an MCU. Each endpoint terminal is then responsible for selecting among the incoming streams and performing its own audio and video processing functions. If an MCU is included in a de-centralized system, it is used as a bridge between the multicast environment and a separate unicast environment.